Phase difference between points on wall

[songoku]

Homework Statement

Parallel water waves of wavelength 10 m strike a straight sea wall. The wavefronts make an angle 30^o with the wall. What is the difference in phases between the waves at two points 5 m apart along the wall?
a. 30^o
b. 45^o
c. 90^o
d. 156^o
e. 180^o

Homework Equations

Δφ = Δx / λ

The Attempt at a Solution

I don't think I can just put Δx = 5 m and calculate using the formula but I have no idea how to start. How can the angle given being used to find the phase difference?

Thanks

 

[haruspex]

How can the angle given being used to find the phase difference?

Draw a diagram of a wavefront striking a point on the wall. Now look at a point 5m further along the wall, which the wavefront hasn't reached yet. How far will the wave need to advance to reach that point?

 

[songoku]

Let me try:
the wave needs to advance for 2.5 m (5 sin 30^o) so the phase difference = 2.5 / 10 rad = 90^o

Is that correct? Thanks

 

[haruspex]

Let me try:
the wave needs to advance for 2.5 m (5 sin 30^o) so the phase difference = 2.5 / 10 rad = 90^o

Is that correct? Thanks

90° is correct, but you mean 2.5/10 (2π) radians.

 

[songoku]

90° is correct, but you mean 2.5/10 (2π) radians.

Yes, my bad

Thank you very much haruspex